1. Field of the Invention
The present invention relates to fluidized bed reactors wherein solids are contacted with gases promoting chemical and physical reactions to the solids, and involving heat, mass and momentum transfer, and transport phenomena.
2. History of the Invention
Fluidization refers to a condition whereby fine solid particles that may be inert, catalysts, or reactants, with respect to a fluid or gas passed therethrough, are suspended within a reactor vessel by velocities of the fluid or gas controlled to cause expansion and formation of a dense phase bed of the solids displaying random movement and mixing of particles. Under desired conditions, bed temperatures and compositions rapidly equalize, bubbles form, coalesce, collapse and reform, and the bed assumes a fairly distinct level, having the appearance of a boiling liquid with hydrostatic and hydrodynamic properties.
The popularity and wide industrial choice of fluidized bed reactors derives from their simplicity in design, concept and operation. Although external mechanical means are required for metering gases and solids, internal mechanical moving parts are seldom involved. Thus, when properly designed, reliability and on-line availability are generally excellent. Such reactors present significant advantages in chemical kinetics and heat and mass transfer that are seldom approached by other types of reactors. The ease and preciseness of control over bed temperature and composition is excellent because of near perfect mixing and rapid heat transfer. Many temperature-sensitive materials can be treated in fluidized bed reactors that are excluded from processing in rotary kilns and in fixed and moving bed reactors. Moreover, small particle sizes of fluidized bed solids expose enormous surface areas for gas-solids contact that promote rapid reaction and heat and mass transfer rates in contrast to limited surface areas of large particles and agglomerates as required for the other types of reactors.
Fluidized bed reactors have no particular or inherent design limitations in regard to pressure. Systems operating at many atmospheres are common, and reaction rates are often accelerated by pressure. In contrast, moving bed reactors and rotary kilns are essentially low-pressure systems, generally limited by complex sealing devices to prevent leakage of gases. Fluidized bed reactors also lend themselves to the economic advantages of continuous rather than batch processing, largely deriving from the ease of handling solids as liquid streams. Their only significant disadvantage is that the many advantages provided by rapid solids mixing and bed uniformity also prohibit countercurrency in a single bed. In practice, however, this disadvantage is overcome by multi-bed reactor designs as are common in the industry.
All fluidized bed reactors include a constriction plate to support the bed, separate it from a windbox wherethrough a gas or like combustible fluid is passed, and to distribute the gas from the windbox uniformly into the bed.
A properly designed constriction plate is essential for assuring uniform gas distribution across the reactor's entire area and through the volume of a fluidized bed. When this is accomplished, in sharp contrast to channeling and bypassing of gases as in other reactor types, operational results often improve on scaleup because wall-effects are minimized and short-circuiting of feed solids directly to discharge decreases with size increases. This fortunate result derives on scaleup from obtaining similar or better mixing characteristics, degrees of gas-solids contact, solids and gas retention times, and thus more favorable chemical reaction and mass and heat transfer rates.
In summary, there are essentially no process design constraints that prohibit scaleup of fluidized bed reactors to massive sizes and productive capabilities. However, as discussed hereinbelow, there are practical mechanical design problems associated with the high temperature and pressure properties of metallic materials that limit scaleup of constriction plates, the present invention providing solutions to many of these constraints. Additionally, limitations also exist on the ability to uniformly introduce and distribute gaseous, liquid, and solid reactants and fuels into high temperature fluidized beds, which limitation the present invention also addresses.
3. Prior Art
Process conditions and the economics of plant capacities dictate particular design features of fluidized bed reactors. However, aside from these factors to avoid a multiplicity of small units, reactor diameter and scaleup have heretofore been dictated by constriction plate mechanical designs as affected by limitations imposed by high temperatures, the location of such temperatures within a reactor, and the physical properties of metallic materials of construction. In this context, high temperatures refer to those substantially in excess of 538.degree. C. (1000.degree. F.) at which the strength of carbon steels and even stainless and alloy steels rapidly decrease, thus curtailing their use as structural materials according to heretofore conventional constriction plate design criterion. Location refers specifically to the windbox through which gas or other combustible fluid is distributed into the bed because the temperature therein establishes the temperature the constriction plate will attain during operation. These considerations have resulted in three general types of constriction plates for three levels of windbox temperatures encountered.